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Old 11-05-2012, 01:29 AM   #1
kamikazekyle
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Join Date: Feb 2007
Posts: 531
Default Circuit monitoring question

Despite beign the noob I am in electronics and circuits, I'm trying to whip up a circuit that lights up a 10 digit LED bar graph from a thermal sensor. That, in of itself, isn't a big deal. A resistor here and there, and off we go.

The problem I've run in to is that the probe in question is already part of another circuit, which is monitored via a third party control unit. I'm trying to find a way to tap into the existing circuit with as little effect on the existing circuit as possible. Right now, maximum variance is +/-4% ohms on the received input at the controller for the current circuit (baseline is 2460 ohms for a 20C reading to 142 ohms at 110C, exponential). I think it runs off a 3V circuit, though I don't have that spec at hand at the moment (just the resistance values).

I could run an entirely new sensor just for my circuit, but that'd triple the cost as I'd have to use kinda pricey rugged sensors. I'd prefer it if I could simply tap the existing sensor feed. I'm thinking of maybe using a volatile digital potentiometer, having its wiper somehow controlled by the voltage of the existing circuit. My circuit will be self powered, so my biggest concern in that case is the wiper causing too much resistance on the existing line (maybe a tiny voltage booster circuit to overcome the natural potentiometer resistance?). That, and I have near zilch experience in EE with ICs and only rudimentary experience with analog circuits. I'm not sure how I'd wire it up.

Another idea I just had was to use some transistors and/or comparator ICs. But I still worry about excessive resistance, even if the components are powered external to the existing circuit.

Finally, I might consider just running my circuit 100% in line with the existing one. Intercept the output from the thermal sensor, use it's value to light my bar graph, then use a voltage booster circuit or IC (or some creative math and component use in my circuit) to ensure that the output back to the control unit is kosher.

Any ideas/opinions/tips? My last option seems the best, though most complex and I dunno how the main controller would react to a PWM voltage booster when it expects nothing but a resistive thermal sensor.

Thanks for the help.
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Old 11-05-2012, 08:02 AM   #2
harrkev
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Hmmmm. I am more of a digital expert, but the first thing that pops into my head is to get a couple of op-amps. Set each to unity gain, and hook up the inputs to each end of the sensor. Op-amps typically have a very high input impedance (get the one with the highest impedance you can find). Then, you have a perfect copy of the two leads of the device, and you can do whatever you want with those voltages.

Note that if one side of the sensor is connected directly to power or gnd, then you have half as much work to do.

Is the sensor a thermocouple of a thermistor?
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Old 11-07-2012, 08:20 PM   #3
DDR4
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Location: Mississauga, ON
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Can you provide a schematic?
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Old 11-08-2012, 11:29 AM   #4
harrkev
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A quick google image search of "voltage follower op amp" turns up this:

http://forum.allaboutcircuits.com/sh...ad.php?t=62562

This type of circuit just "copies" a voltage while having almost no effect on the ciruit being monitored at all. Assuming that the sensor does NOT have one side connected to power or ground, you will need two op-amps, one to copy the voltage on each side of the sensor. Then, you can use the two outputs to drive anything else you want. Note that you can easily get op-amps with two or four to a single chip. Since this is all very low-frequency stuff, you can really use any one that you want, but you do want one to get as high an input impedance as possible -- which means that it will have as little effect on the source circuit as possible.

If you want something immediately, you should be able to find this near you quite easily:

http://www.radioshack.com/product/in...alue=IC-Analog

This is a quad op-amp. I would suggest tying any unused inputs for the other two amps to gnd through some 1K resistors. You do NOT want them floating, otherwise the output of those unused amps may swing around, and cause power supply problems. Or, you could use those amps for another part of your circuit.
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